1. Field of the Invention
This invention relates to collimating apparatus for controlling the shape and size of the beam of radiation such as an X-ray beam and more particularly for means for controlling secondary radiation in such apparatus.
2. Description of the Prior Art
For diagnosis and treatment of the human body, it is desirable to provide a beam of X radiation which impinges upon the subject only in a desired specific location and which is confined to the specific area of interest. To this end, collimating devices utilizing one or more sets of adjustable shutters are utilized. Such apparatus is well known to those skilled in the art.
In an X-ray generating tube, a beam of electrons is directed or focused towards a small area or focal spot on a rotating anode disc. For various physical phenomenon, the focusing of this beam of electrons is not achieved completely. As a result, X-rays are generated and emitted not only from the desired focal spot, but also from other peripheral areas of the anode disc as well as from some of the disc mounting parts, e.g. the stem and retaining nut. Thus in practice, secondary radiation often referred to as off focus radiation, is emitted from all X-ray generating tubes at angles different from the primary beam.
U.S. Pat. No. 2,722,611 issued to W. H. Haupt, discloses a collimating apparatus of the type described, and additionally includes a cone which is located between the X-ray tube head and the shutter mechanism. However, to effectively control and restrict the emission of secondary radiation, it is imperative that the geometrical location of the near shutters in a double tier shutter mechanism, be located as close to the focal spot as physically permissible. In attempting such an arrangement, however, the physical space limitations of prior art apparatus results in undesirable amounts of secondary radiation passing through the restricting shutter mechanisms.